The invention relates to a method and apparatus for providing controlled rate distribution of liquids or liquid suspensions of fine solids for such agricultural purposes. Such purposes include fertilization and growth regulation of crops and eradication of unwanted plant life in a crop field.
Wet blade agricultural liquid distribution systems rely on the rapid uptake of liquids that occurs in freshly cut plants at the wound. An order of magnitude reduction in the concentration of agricultural liquids is possible by applying the liquids essentially simultaneously with cutting. To accomplish this, liquid dispersal mechanisms have been incorporated into mowers. The advantages of this system are described in pending patent application PCT/US96/13362.
Peristaltic pumps are pumps of choice when flow rates are moderate and either corrosive, toxic or sterile liquids are pumped. The peristaltic pump utilizes the fact that the liquid to be pumped resides in a flexible tube. A length of tube is placed inside a rigid semi-cylinder. See FIG. 1. A rotating hub at the center of the cylinder drives a roller against the tube causing a flexure in the tube to travel in the direction in which the impeller hub rotates. For clarity, an impeller with a single roller is illustrated. As the flexure of the tube is relieved after the compression of the passing roller, a partial vacuum forms, drawing liquid from the intake side. The flow rate of liquid through the tube depends on the size and elastic properties of the tube, the hub rotation speed, the number of rollers, the viscosity of the fluid being pumped and the amount of tube compression. Tube compression depends on the size of the roller and the impeller arm dimension relative to the center of the cylinder.
This pumping system is preferable for use with vegetation control liquids because it can handle a variety of liquids and because human contact with some liquids is undesirable for safety reasons. If a conventional pumping system were used for both herbicides and fertilizers, extensive flushing of the system would be required between application of the two materials. With a peristaltic pump, the tubing associated with each chemical can be changed. The pump mechanism never directly contacts the pumped material. The part most subject to wear is the tubing, which is inexpensive to replace when necessary.
Field experience with this system resulted in the identification of several problems. Normally, peristaltic pumps run at a constant rate. In this agricultural application, the rate must change as the mower slows to turn or in response to variable terrain. A first solution of this problem was to replace the motor driving the peristaltic pump with a stepper motor. Step pulses drive the motor in harmony with the motion of the mower. With this improvement, wet blade distribution was practical. With varying materials, large changes in flow rates are often needed. To achieve integer multiples of a base flow rate for a given tube size, a plurality of tubes may be stacked in the cylinder. A tube coming from the source tank can be split into a manifold. The stack of tubes may be introduced into the cylinder. The number of tubes in the stack are limited by the height of the cylinder and roller. The output side of each tube can be distributed to appropriate nozzles or recombined with a manifold for material application.
Different size tubes can be used to achieve flow rates intermediate to the multiple tube manifold approach. A larger tube and compatible smaller diameter roller can be used to adjust flow rates.
Currently available peristaltic pumps are not designed for the field conditions encountered in the wet-blade distribution of agricultural chemicals. Even with the stepping motor regulator, there are problems in maintenance, set up and fine flow regulation. To make modifications in the field, the peristaltic pump is disassembled. The disassembly/reassembly process is problematic even for simple periodic maintenance like pump lubrication. Access to the pump mechanism requires removal of multiple machine-screws. Reassembly requires thumb screw adjustment of ferrules through which the tube is threaded. Too much pressure on the thumb screw causes the tube to constrict changing pump characteristics; too little pressure causes loss of control of the loop size within the pump. In addition, the screw mars the tube surface and if over tight, cuts the surface unacceptably. Disassembly to change tube size requires removing many small machine-screws.
The principal object of this invention is to provide an apparatus for metering agricultural chemicals in a wet blade distribution system that can be easily set up in the field to handle a wide variety of distribution rates and fluid viscosities.
Another object of this invention is to provide an apparatus that can react to various mower speed over the terrain.
Another object of this invention is to provide a modular peristaltic pump unit that is readily interchangeable.
A further object of this invention is to provide a peristaltic pump apparatus that is easy to maintain in the field.
Another object of this invention is to provide an improved method for metering agricultural chemicals in a wet blade distribution system.
The invention consists of a modular peristaltic pumping system that uses an associated microprocessor-based servomechanism for controlling fluid chemical distribution. The microprocessor stores: the number of modules attached, the roller axis setting, the elastomeric properties of the tubes used, the fluid properties of the material being pumped and the desired distribution rate for the agricultural objective. Real time inputs to the processor include the ground speed of the mower and the angular velocity of the impellers. From this data, the required impeller speed can be continuously calculated. This data and the characteristics of a DC motor are continuously used to calculate a data stream representing the needed motor speed. A digital-to-analog converter with appropriate buffering then drives the motor. The motor shaft drives the impellers in each pump module. The peristaltic modules are mounted on a frame. The motor is mounted to the frame. The motor shaft ends in a bit, similar to a screwdriver bit, that engages a mating slot in the first module. The shaft of the module engages the module""s impeller and exits the module in a bit that is appropriate to engage a mating slot in the next module. Thus, modules can be ganged to permit an integer number of base distribution rates. Modules can be assembled on the frame without tools. Modules may be lubricated without tools. Modules can be disassembled without tools to change the tube or modify the effective impeller arm length.
Alternatively, a transmission may be provided between the motor shaft and the drive shaft of the peristaltic pump stack. The transmission may consist of a gear train or one or more pulleys and belts. In this case, the drive shaft of the transmission ends in a bit that engages a mating slot in the first module.